Purification of carboxylic acids by plural stage distillation with side stream draw-offs

ABSTRACT

A process for the removal and recovery of halogen components and the drying of aqueous monocarboxylic acids, particularly those produced by the reaction of an alcohol or olefin and carbon monoxide in the presence of a catalytic system comprising a Group VIII metal component and a halogen component containing bromine or iodine and at least a portion of the halogen component being either an alkyl halide or a hydrogen halide. The process comprises introducing a monocarboxylic acid stream containing water and the halogen contaminant into a first distillation column, removing an overhead fraction containing primarily alkyl halide, removing a bottoms fraction from said first distillation column containing the hydrogen halides present in said column, taking a stream from the middle portion of said first distillation column and introducing it into the upper half of a second distillation column, removing an overhead fraction consisting primarily of the water charged thereto, taking a stream from the middle portion of the second distillation column and recycling such stream to the lower half of the first distillation column, said stream containing essentially all of the hydrogen halide present in said second distillation column, and removing a product acid stream at or near the bottom of said second distillation column, the product acid stream being essentially dry and substantially free of the halogen components charged to the first column. The method is particularly applicable to the removal of water and iodine-containing compounds from acetic and propionic acids.

United States Patent 1 Eubanks et al.

[ PURIFICATION OF CARBOXYLIC ACIDS BY PLURAL STAGE DISTILLATION WITHSIDE STREAM DRAW-OFFS [75] Inventors: Lloyd S. Euhanks, Texas City;

Krouse S. McMahon, El Lago; John T. Payne, Texas City, all of Tex.;Frederick E. Rosenherger, Des Peres, Mo.

[73] Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: Nov. 10, 1971 21 Appl. No.: 197,434

[52] US. Cl 203/71, 203/15, 203/16,

3,507,755 4/1970 Bitners et a'l.. 203/98 3,518,165 6/1970 Ward 203/993,700,566 10] 1972 Bellinger et 81.. 203/99 3,210,271 10/1965 Byerly eta1 203/82 Primary ExaminerWilbur L. Bascomb, Jr. AttorneyThomas B.Leslie et a1.

14 T/IB 29 1 Oct. 30, 1973 [57] ABSTRACT A process for the removal andrecovery of halogen components and the drying of aqueous monocarboxylicacids, particularly those produced by the reaction of an alcohol orolefin and carbon monoxide in the presence of a catalytic systemcomprising a Group VIII metal component and a halogen componentcontaining bromine or iodine and at least a portion of the halogencomponent being either an alkyl halide or a hydrogen halide. The processcomprises introducing a monocarboxylic acid stream containing water andthe halogen contaminant into a first distillation column, removing anoverhead fraction containing primarily alkyl halide, removing a bottomsfraction from said first distillation column containing the hydrogenhalides present in said column, taking a stream from the middle portionof said first distillation column and introducing it into the upper halfof a second distillation column, removing an overhead fractionconsisting primarily of the water charged thereto, taking a stream fromthe middle portion of the second distillation column and recycling suchstream to the lower half of the first distillation column, said streamcontaining essentially all of the hydrogen halide present in said seconddistillation column, and removing a product acid stream at or near thebottom of said second distillation column, the product acid stream beingessentially dry and substantially free of the halogen'components chargedto the first column. The method is particularly applicable to theremoval of water and iodinecontaining compounds from acetic andpropionic acids.

7 17 Claims, 1 Drawing Figure W490 I II\ D 20 34 2/ v 26 I0 V v y 24 VPAIENIEDucI 30 ms T/IB PURIFICATION OF CARBOXYLIC ACIDS BY PLURAL STAGEDISTILLATION WITH SIDE STREAM DRAW-OFFS BACKGROUND OF THE INVENTION Thepresent invention relates to the purification of carboxylic acids. Moreparticularly, the present invention relates to the purification ofproduct streams of carboxylic acids produced by a catalytic system containing halogen components and which streams contain residual halogencomponents and water.

There have recently been proposed several processes for the productionof carboxylic acids by the reaction of alcohols or olefins and carbonmonoxide in the presence of catalytic systems containing (1) a GroupVIII metal component such as a component containing ruthenium, rhodium,osmium, iridium, platinum, palladium, cobalt, nickel, etc. and (2) ahalogen component, primarily a bromine or iodine containing component.Generally, the halogen component in the catalytic system is present asan alkyl halide such as methyl iodide or a hydrogen halide such ashydrogen iodide.

While the carboxylic acids produced by the above described processes aregenerally of relatively high purity as far as other organic by-productsare concerned, they do contain water and relatively small amounts ofhalogen components as contaminants. In order that the carboxylic acidsproduced by such processes can be utilized in further reactions andother uses they must 7 generally be freed from any water which ispresent as well as the small amounts of halogen contaminants present.When freed from such impurities the carboxylic acids thus produced areadmirably suited for practically all uses in commerce and industry ofsuch carboxylic acids and find a ready market therein.

SUMMARY It is an object of the present invention to provide aprocess forthe purification of carboxylic acids.

A further object of the present invention is to provide a process forthe removal of water as well as the removal of halogen-containingimpurities from streams of carboxylic acids. 7

A still further object of the present invention is to provide a processfor the removal of halogenated impurities from carboxylic acids, thecarboxylic acids having been prepared by the reaction of an alcohol orolefin and carbon monoxide in the presence of a catalyst systemcomprised of a Group VIII metal-containing component and ahalogen-containing component.

Another object of the present invention is to provide for the recoveryof the halogen components of such a halogen-containing catalyst systemfor reuse in such system.

Another object of the present invention is to provide a process for thepurification of acetic and propionic acids.

These and other objects of the present invention will become apparentfrom the drawing, description given herein and the appended claims.

In the purification process of the present invention a stream ofcarboxylic acid containing from 2 to 10 carbon atoms and containing ascontaminants water and certain halogenated materials, is introduced intoa distillation zone. Alkyl halides present along with a portion of thewater present are removed as an overhead stream from this column whilesubstantially all hydrogen halides present are removed in the bottomsstream. Both these streams can be stored, disposed of or, recycled tothe prior catalytic production process. A stream from the 'rniddleportion of the first distillation zone is removed from said zone andintroduced into a second distillation zone wherein there is removed asan overhead stream the remaining portion of the water present, a streamfrom the middle portion of the second distillation column is recycled tothe first column and a product acid stream essentially dry andsubstantially free of the halogen components is removed at or near thebottom of said distillation zone..

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE is a schematic flow diagramof the purification process of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The description of the manner ofcarrying out the purification process of the present invention followswith specific reference to the process flow diagram in the drawing. Astream of carboxylic acid to be purified, in either liquid or vaporform, is introduced via line 10 into column 11 intermediate the endsthereof, and preferably at a point in the lower half of column 11. Thebottoms stream of condensed acid still containing some water andsubstantially all the hydrogen halide component which has beenconcentrated in such bottoms is removed-via line 12 and preferablyrecycled to the reactor producing the acid product stream. An overheadstream is removed via line 13 and condensed in condenser 14, thecondensed stream passing via line 15 to phase separation vessel 16. Inphase separator 16 the uncondensed volatile material present, consistingsubstantially of carbon monoxide with a few percent of vaporized alkylhalide, can be either vented or alternately returned to the precedingprocess via line 18. The liquid present separates into a lighter waterphase containing a minor amount of carboxylic acid and very smallquantities of condensed alkyl ester and alkyl halide, and a heavierphase containing substantially all the liquified alkyl halide and only afew percent of water, ester and carboxylic acid. This heavier, stream isconcentrated in the sump 16A of separator 16 and is removed via line 17for return to the prior production process. As shown, provision is madevia line 19 for recycle of a portion of the lighter, predominantly waterphase to distillation column 11 to serve as reflux, and for return ofthe remaining, usually the minor, portion of such lighter phasevia line19a to the prior production process.

The provision of separation vessel 16 and the removal thereby of themajor proportion of the water and alkyl halide present in the initialfeed from the overhead stream from the first distillation column resultsin a particularly efficient purification process. At least twoprimeadvantages to such process are achieved. First, the alkyl halidecomponent essential for the operation of the prior catalytic productionprocess is concentrated, separated and returned to such process at theearliest stage of purification of the carboxylic acid stream.Furthermore, the removal from the purification process of the majorproportion of the water as well as the alkyl halide present enables theremainder of the drying and purification of the carboxylic acid productto be carried out in a second distillation column of more economicalsize. Alternatively, it provides the ability todry and purify a greaterthroughput of acid in such second distillation column than wouldotherwise be possible without such removal.

A stream of principally carboxylic acid and water is withdrawn from themiddle portion of distillation column 11 and passes via line 20 to asecond distillation column 22. If required in order to maintain liquidreflux and a minimum water content in column 11 bottoms provision ismade by the junction shown for recycle of a portion of the side streamvia line 21 to below the plate at which the side stream was taken.Thefeed stream entering column 22 via line 20 is introduced above themid point of column 22. An overhead stream from column 22 is removed vialine 28 and condensed in condenser 29, the condensed stream passing vialine 30 to separation vessel 31 In this separator 31 any remainingvolatile material, generally only carbon monoxide, is either vented orrecycled to the production process via line 32. The relatively dilutecarboxylic acid liquid solution present in separator 31 is split asshown to supply a portion as reflux to distillation column 22 via line33 and the other portion of said dilute: solution is withdrawn via line34 for either disposal or recycle to the earlier process.

A stream is withdrawn via line 25 from the middle portion ofdistillation column 22 of controlled water content. For example, bycontrol of the temperature under the pressure condition adopted of apoint in column 22 near the point of removal of the side stream in line25 a controlled small percentage of water can be maintained in such sidestream. By virtue of the solubility of hydrogen halide in liquidcarboxylic acid water mixtures containing from about three to abouteight percent of water and the increasing volatility of such hydrogenhalide in liquid mixtures of lower water content, a peak concentrationof hydrogen halide occurs in the middle portion of column 22 where thecomposition of carboxylic acid and water liquid mixtures ranges fromabout three to about eight percent water. If a liquid side stream ofcarboxylic acid and water of such composition is withdrawn via line 25from a point at or near such peak concentration of hydrogen halide, thensubstantially all the hydrogen halide present in column 22 will beremoved. If desired to provide for withdrawal of side stream 25 undervarying conditions of temperature and pressure in column 22, a pluralityof valved take-off points may be provided, not shown, from a series oftrays in the middle portion of column 22.

The side stream is passed via line 25 and recycled to the lower portionof distillation column 1 l. A sufficient concentration of water ismaintained in the lower portion of column 11 such as by provision ofreflux through line 21 to result in maintainance of substantially all ofthe hydrogen halide recycled in stream 25 in solution in the bottoms ofcolumn 11, which bottoms is withdrawn via line 12. So long as theconcentration of water in'the bottoms of column 11 is at least aboutfour percent or more in the case of acetic acid, substantially all thehydrogen halide will remain in solution therein. There can also beprovided, as shown, valve 26 and line 27 which allows the alternative ofwithdrawal and storage of this carboxylic acid, water and hydrogenhalide stream in line 25 for later treatment when the operation of theprocess supplying the original acid mixture may require same. As aresult of treatment and distillation in column 22, a purified acidbottoms collects therein from which is withdrawn via valve 23 and line24 the product carboxylic acid stream. This stream is essentially drycarboxylic acid and substantially free of the halogen components presentin the original crude acid stream charged to the system via line 10.

It will be apparent from the description that there is virtually nowaste from the present purification process, all the streams withdrawnbeing adaptable to recycle to the reaction process producing the acidmixture which ispurified. Thus the present process is extremelyeconomical in the recovery of all active halogen catalyst components aswell as unreacted initial reactants, such as carbon monoxide, forsubsequent reuse in the catalytic production of additional carboxylicacid.

The carboxylic acids which may be purified by the process of the presentinvention comprise monocarboxylic acids containing water and at leastone halogen contaminant as an impurity irrespective of the process bywhich such acids may have been produced. The present process is mostuseful with carboxylic acids of 2 to 10 carbon atoms, preferably 2 to 4carbon atoms. The process is particularly applicable to the purificationof acetic and propionic acids. Specifically, the monocarboxylicacidstream can be one produced by the reaction of an alcohol or olefinand carbon monoxide in the presence of a catalyst system containing aGroup VIII noble metal component and a halogen component, usuallybromine or iodine. Included among the Group VIII noble metals areiridium, rhodium, platinum, palladium, cobalt, nickel, osmium,ruthenium, etc. At least a part of the halogen component is usuallypresent in the form of an alkyl halide and/or a hydrogen halide. Theprocess of the present invention is particularly applicable to thepurification of acetic and propionic acids containing halogenatedimpurities and more particularly to the acetic and propionic acidstreams which contain iodine contaminants such as alkyl iodide or.hydrogen iodide. Such acetic and propionic acid streams are produced bythe reaction of, respectively, methanol and carbon monoxide or ethanolor ethylene and carbon monoxidein the presence of a catalyst systemcontaining iridium, rhodium, platinum, palladium, osmium, or rutheniumand alkyl iodide and/or hydrogen iodide.

In the present purification process briefly described above the firstdistillation zone can comprise any distillation column normally used forseparation and purification and can be of either the packed or platetype or can be a combination packed plate type. Generally, the firstdistillation zone will comprise a plate type column having from 2 to 25trays and preferably from 5 to 20 trays and in an especially preferredembodiment employs sieve trays although other type trays such as bubblecap and ballast may be employed.

The second distillation zone can, as in the case of the firstdistillation zone, comprise any distillation column normally employedfor the separation and purification of fluids. It can also be of thepacked or plate type or a combination of the two. Generally, the seconddistillation zone will comprise a plate type column having from 10 totrays and preferably from 20 to 60 trays. Although bubble cap trays andballast trays may be employed in the column comprising the seconddistillation zone, it is preferred that sieve trays be used.

The associated condensers and liquid separation vessels employed witheach of the distillation columns described are of generally conventionaldesign and manufacture. They can be of the open vessel type or cancontain baffles or other means for suppressing surging if desired. It ispreferred that the liquid phase separation vessel associated with theoverhead stream from the first distillation zone have provision for aninternal or external sump-for collection of the concentrated heavyliquid phase separated therein.

As will be recognized, various pumps, compressors, reboilers, separationvessels, etc., normally employed in carrying out chemical processes canbe employed in the process described herein. Since these do not formpart of this invention, the details of their use in various phases ofthe process description have not been included.

The temperatures and pressures employed in the two distillation zonesdescribed above will vary considera bly depending upon the particularcarboxylic acid stream being purified. As a practical matter these zonesare most often operated at pressures from approximately atmospheric to100 psig, although subatmospheric pressures may be employed if desiredas well as super-atmospheric pressures well in excess of 100 psig.Temperatures within the zones'will normally lie between the boilingpoint of the particular carboxylic acid being purified and approximatelythe boiling point of water at the pressure of the zone.

When employing the process of the present invention to purify acetic orpropionic acids, the two distillation zones are usually operated atpressures within the range ofO to 60 psig, preferably at pressureswithin the range ofO to 45 psig. At these pressures, the bottomstemperatures of the two zones generally will be within the range of fromapproximately the boiling point of the acid at the pressure employed toas high as 165C or higher, but preferably below about 165C. Thetemperatures at the top of the distillation zones likewise range fromthe boiling point of the acid being purified at the pressure employed toas low as 100C. The temperatures and pressures of the two distillationzones may be the same or different, but most often the temperatures andpressures of the second distillation zone are maintained at somewhathigher values than the first distillation zone.

While the point of introduction of the feed stream to the firstdistillation zone can vary intermediate the ends of the zone, the feedstream usually is introduced into the lower half of that column andpreferably into the lower third thereof. While the stream fed to thesecond distillation zone can be removed from any plate in the middleportion of the first distillation zone above the' point of introductionof the feed stream thereto and below the point of overhead reflux, it ispreferably removed at an intermediate plate wherein there is present anapproximately 80/20 percent acid/water composition at the temperatureand pressure selected for operation of that zone.

The feed stream to the second distillation zone just detailed also canbe introduced anywhere in the upper half of that zone. Generally, thisfeed is introduced into the upper one-third of the second distillationzone. The side stream taken from the second distillation zone forrecycle to the first distillation zone is taken from a plate in themiddle portion of the second distillation zone so selected at thespecific temperature and pressure of operation that the concentration ofhydrogen halide is near, and preferably at, the highest of the entirezone on that plate, since the purpose of this recycle stream of acid andwater is to remove all of the remaining hydrogen halide present in thesecond distillation zone. The product stream removed from this seconddistillation zone can be removed at any point in the lower onethird andpreferably from the lower one-tenth of this zone. If a fully condensedliquid product is desired the most desirable point for withdrawal of theproduct stream from this second zone yielding the dryest acid product isachieved by taking a bottoms stream from the second distillation zone.Alternatively, if a product stream containing no trace of metallichalide impurities is desired, then such product stream can be withdrawnin vapor form from a point above the liquid level of the second columnbottoms. A convenient point, not shown, is just above or below thelowest plate in the second distillation column.

The purified product acid stream thus realized is suitable for mostuses, both commercial and others, of the various monocarboxylic acidspurified by the presently described process. This is particularly truewith reference to the preferred acetic and propionic acids so purified.However, if it is desired that the carboxylic acid products be virtuallycompletely free of any halogen contaminants because of the very severerequirements of certain uses as highly purified reactants in catalyticsystems in which the catalyst is most sensitive to the presence of eventhose trace amounts of halogen impurities which may remain in the acidproducts purified by the present process, then these acid products canbe submitted to further, even more strenuous purification by additionaltreatments or processes. However, such additional treatment or processesform no part of the present invention and require no discussion herein.

In the application of the present process to the preferred acetic andpropionic acids the percentages of the total feed to the firstdistillation zone which are removed in the various fractions taken fromthat zone can vary somewhat. Generally, the overhead stream removed fromthe'first distillation zone and either recycled to supply reflux theretoor recycled to an earlier stage of the production process employed, willrange from approximately 30 to 50 percent, and preferably 37 to 47percent by weight, of the total feed to the first distillation zone.Likewise, the proportion of the total feed to the first distillationzone represented by the feed stream to the second distillation zonetaken from the middle portion of the first zone will generally range tqma zqqt9lq @91 69. .B IFFI L an p f y from about 52 to about' 58 percent,by weight of the total feed to that first zone. Thus the total take-offof hssslw s uqps streams wil a q or m .1 99.5 percent, and preferablyfrom to 99 percent, of the total feed. The percentage of the total feedto the first distillation zone removed in the bottoms stream recycled tothe earlier production process will range from about 0.5 to about 6percent by weight of the total feed to that zone, and preferably fromabout 2 to 5 percent thereof.

The percentages of the total feed to the second distillation zonelikewisecan be varied to some extent. The percentage of the feed to thesecond distillation zone represented by the overhead stream can bevaried since in the case of that stream the portion recycled as refluxto the upper portion of the second distillation zone can be adjustedconcurrently. This overhead stream generally represents from about 20 toabout 50 or more percent, and preferably from about 25 to about 35percent by weight of the total feed to the second distillation zone. Thepercentage of the total feed to the second distillation zone representedby the side stream taken from the middle portion of that zone andrecycled to the first distillation zone represents a very'smallproportion of the feed and generally ranges from about one to abouteight percent, and preferably, from about three to about five percent,by weight of the total feed to the second distillation zone. Thewithdrawal rate of the bottoms stream of purified carboxylic acidproduct is not limited and care need only be taken to retain sufficientliquid bottoms in the second distillation zone to accommodate the heatinput from a reboiler or other heating means and to avoid starving thisbottom zone to dryness.

To demonstrate the effectiveness and to illustrate the application ofthe process of the present invention, the following non-limitingexamples are set forth.

EXAMPLE A stream of acetic acid, water and halogen components includinghydrogen iodide and methyl iodide which was produced by a noble metaland iodide catalyzed reaction of methanol with carbonmonoxide was driedand purified of the halogen components in a purification and recoverysystem as illustrated in the drawing. The first distillation column wasa packed-plate coiumn of six inch diameter and approximately ninetheoretical plates, while the second distillation column was of thepacked type of six inch diameter and approximately 35 theoreticalplates. The acid stream treated had an approximate composition by weightof to percent water, to percent methyl iodide, trace quantities of lessthan 500 ppm hydrogen iodide and the remainder acetic acid. A continuousrun of five days duration, was monitored and samples of each of thestreams set out in Tables 1 and 2 below were analyzed at approximately 6hour intervals during each of the 5 days with intervals of 12 hoursbetween each days sampling and analysis. The streams from each columnwere controlled to the indicated temperatures. During operation theoverhead pressures in each of the columns were controlled toapproximately 6.7 psig in the first column and approximately 24 psig inthe second column. The results of the analysis of the samples of eachstream monitored during the continuous run are set out in Tables 1 and 2below, which refer respectively to the streams withdrawn from the firstand second distillation columns.

TAB LE 1. First column Bottoms Side stream Overhead reflux (light phase)Tem- Composition Te'm- Composition Tem- Composition perature 4 peratureperature H O wt. [-1 0 wt. Mel wt. H O wt. Mel wt. Day Time percent H1p.p.m. percent percent H1 p.p.m. percent percent HI p.p.m.

TAB LE -2.Sec0nd column Bottoms product Side stream Overhead Tem-Composition Composition Tem- Composition perature perature perature aCH20 H20 wt. 0C H O wt. Mel wt. Day Time p.p.m. Hl p.p.m. percent H1p.p.m. percent percent H1 p.p.m.

1 0800 152 146 6.9 2,060 130 84 1.0 104 1400 152 100 147 4.3 1,040 13169 1.7 55 2000 152 100 1 v 146 6.4 1,360 130 76 2.1 111 2..- 0800 154100 7 146 3.7 1,340 130 61 2.3 92 1400 152 100 7 146 4.8 2,620 130 612.8 134 2000 153 100 8 146 5.5 2,200 131 62 3.1 71 3 0800 152 200 9 1465.9 1,040 131 59 2.3 79 1400 153 6,800 8 147 3.8 1,600 129 2.2' 2000 1542,400 377 146 4.7 1,140 131 66 2.2 121 4 0800 152 170 3 146 7.0 1,380131 t 2.8 95 1400 153 100 I 2 146 4.7 1,990 68 1.5 174 2000 152 100 2146 4.9 1,060 130 63 2.6 131 5 0800 152 100 6 146 5.0 2,260 130 62 2.4212 1400 152 100 10. 5.9 2,260 130 65 3.1 225 2000 152 100 3 145 7.12,630 130 70 2.3 206 M The concentration of methyliodide in bottomsproduct was less than 0.50 p.p.m. in all samples.

In Table 2 above the excess water content of the bott product at AQan21Q0 9f was stalemsents a column upset due to minor flooding. The resultof such excess water content in the bottoms on the hydrogen iodidecontent thereof is evident in the 2,000 sample. Except for the aboveperiod of upset, it is apparent from the above data that the process ofthe present invention produces a bottoms product acid stream essentiallydry and substantially free of the halogen components present in theinitial feed stream of carboxylic acid and water.

What is claimed is:

1. A process for the purification and drying of monocarboxylic acidstreams containing water and halogen contaminants including an alkylhalide and a hydrogen halide, which process comprises a. introducing anacetic or propionic acid stream containing water and said halogencontaminants into a first distillation zone,

b. removing an overhead fraction containing a proportion of the waterand a major proportion of the alkyl halide charged to said zone,'

c. removing a stream from the bottom portion of said first distillationzone containing a major proportion of the hydrogen halide charged tosaid zone,

d. removing a stream from the middle portion of said zone andintroducing said stream into the upper half of a second distillationzone,

e. removing an overhead fraction from said second distillation zonecontaining a major proportion of the water charged to said second z'one,

f. removing a stream from the middle portion of said second zone belowthe point of introduction in (d) and at or near the peak concentrationof hydrogen halide present in said zone,

g. removing a product monocarboxylic acid stream from at or near thebottom of said second distillation zone, the said product acid streambeing essentially dry and substantially free of the halogen contaminantscharged to said first zone.

2. The process of claim 1 wherein the overhead fraction (b) is separatedinto a heavy phase of predominantly alkyl halide and a lighter phase ofpredominantly water.

3. The process of claim I wherein said halogen contaminants compriseiodides.

4. The process of claim I wherein said halogen contaminants comprisebromides.

5. The process of claim 1 wherein said monocarboxylic acid streamcomprises acetic acid and said halogen contaminants comprise iodides.

6. The process of claim 1 wherein the monocarboxylic acid streamcomprises acetic or propionic acid produced by the reaction of analcohol or olefin and carbon monoxide in the presence of a catalystsystem of a Group VIII noble metal component and a halogen component.

7. The process of claim 1 wherein the monocarboxylic acid streamcomprises acetic acid produced by the reaction of methanol and carbonmonoxide in the presence of a catalyst system of a Group VIII noblemetal component and an iodide.

8. The process of claim 1 wherein a portion of stream (d) is recycled tosaid first distillation zone below the point at which stream (d) isremoved.

9. The process of claim 1 wherein the product monocarboxylic acid streamis removed in vapor form from above the liquid level in the bottom ofsaid second distillation zone.

10. The process of claim 1 wherein the product monocarboxylic acidstream is removed from the bottoms of said second distillation zone.

11. The process of'claim 1 wherein said first distillation zonecomprises a distillation column having from 2 to 25 trays.

12. The process of claim 1 wherein said second distillation zonecomprises a distillation column having from 10 to trays.

13. The process of claim 1 wherein the first and second distillationzones are maintained at pressures of from atmospheric to psig and attemperatures of from 100 to I65C.

14. The process of claim 1 wherein the proportion of the total feedstream (a) to said first distillation zone which is removed from saidzone as overhead fraction (b) and middle stream (d) is from 80 to 99.5weight percent of said total feed stream.

15. The process of claim 1 wherein the proportion of the total feedstream (a) to said first distillation zone which is removed from saidzone as bottoms stream (c) is from 0.5 to 6 weight percent of said totalfeed stream.

16. The process of claim 1 wherein the proportion of the feed stream (d)to said second distillation zone which is removed from said zone asmiddle stream (f) is from one to eight weight percent of said feedstream.

17. The process of claim 1 wherein overhead fractions (b) and (e) andbottoms stream (c) are returned to the process for producing saidmonocarboxylic acid stream.

2. The process of claim 1 wherein the overhead fraction (b) is separatedinto a heavy phase of predominantly alkyl halide and a lighter phase ofpredominantly water.
 3. The process of claim 1 wherein said halogencontaminants comprise iodides.
 4. The process of claim 1 wherein saidhalogen contaminants comprise bromides.
 5. The process of claim 1wherein said monocarboxylic acid stream comprises acetic acid and saidhalogen contaminants comprise iodides.
 6. The process of claim 1 whereinthe monocarboxylic acid stream comprises acetic or propionic acidproduced by the reaction of an alcohol or olefin and carbon monoxide inthe presence of a catalyst system of a Group VIII noble metal componentand a halogen component.
 7. The process of claim 1 wherein themonocarboxylic acid stream comprises acetic acid produced by thereaction of methanol and carbon monoxide in the presence of a catalystsystem of a Group VIII noble metal component and an iodide.
 8. Theprocess of claim 1 wherein a portion of stream (d) is recycled to saidfirst distillation zone below the point at which stream (d) is removed.9. The process of claim 1 wherein the product monocarboxylic acid streamis removed in vapor form from above the liquid level in the bottom ofsaid second distillation zone.
 10. The process of claim 1 wherein theproduct monocarboxylic acid stream is removed from the bottoms of saidsecond distillation zone.
 11. The process of claim 1 wherein said firstdistillation zone comprises a distillation column having from 2 to 25trays.
 12. The process of claim 1 wherein said second distillation zonecomprises a distillation column having from 10 to 90 trays.
 13. Theprocess of claim 1 wherein the first and second distillation zones aremaintained at pressures of from atmospheric to 100 psig and attemperatures of from 100* to 165*C.
 14. The process of claim 1 whereinthe proportion of the total feed stream (a) to said first distillationzone which is removed from said zone as overhead fraction (b) and middlestream (d) is from 80 to 99.5 weight percent of said total feed stream.15. The process of claim 1 wherein the proportion of the total feedstream (a) to said first distillation zone which is removed from saidzone as bottoms stream (c) is from 0.5 to 6 weight percent of said totalfeed stream.
 16. The process of claim 1 wherein the proportion of thefeed stream (d) to said second distillation zone which is removed fromsaid zone as middle stream (f) is from one to eight weight percent ofsaid feed stream.
 17. The process of claim 1 wherein overhead fractions(b) and (e) and bottoms stream (c) are returned to the process forproducing said monocarboxylic acid stream.